The present invention relates to a blade for a turbine, e.g. aircraft engine, gas turbine, steam turbine, etc. More specifically, the present invention relates to hollow cavity tip shrouds and the cooling of a turbine blade tip shroud through the use of circulating coolant through the hollow cavity. As a non-limiting example, the invention and its background are described with reference to a gas turbine.
The turbine blades of industrial gas turbines and aircraft engines operate in an extreme temperature environment. The thermal stresses and metal temperatures associated with this environment may decrease the useful operating life of the turbine blades. Cooling the turbine blades, and the component parts thereof, during operation may extend their useful operating life.
Many turbine blades include an airfoil and an integral tip shroud attached to the tip of the airfoil. The tip shroud, which attaches to the outer edge of the airfoil, provides a surface area that runs substantially perpendicular to the airfoil surface. The surface area of the tip shroud helps to hold the turbine exhaust gases on the airfoil (i.e., does not allow the exhaust gases to slide over the end of the airfoil blade) so that a greater percentage of energy from the turbine exhaust gases may be converted into mechanical energy by the turbine blades. Tip shrouds thusly improve the performance of the gas turbine engine. Further, it is desirable to have the entire outer surface of the airfoil covered by a tip shroud. However, tip shrouds and the connection they make to the airfoils become highly stressed during operation because of the mechanical forces applied via the rotational speed of the turbine. When these mechanical stresses are coupled with the thermal stresses and metal temperatures associated with extreme high temperature environment of the turbine, it becomes a challenge to design a tip shroud that will perform its intended function over the entire useful life of the airfoil.
Two possible methods of resolving this issue are to either: 1) reduce the mechanical stresses applied to the tip shrouds by reducing their weight, or 2) reduce the metal temperatures experienced by tip shrouds. As to the first, one common method for reducing tip shroud weight is to “scallop” (i.e., remove an indentation or a portion of) the overhanging tip shroud. The reduction in tip shroud material results in a reduction of the load applied to the connection made between the tip shroud and airfoil during operation. However, decreasing the surface area of the tip shroud through scalloping comes at a cost as it decreases the performance of the turbine engine because a tip shroud of less surface area has a decreased ability to hold the turbine exhaust gas on the turbine airfoil (i.e., more of the exhaust gases slide over the top of an airfoil that has a tip shroud of reduced surface area). In regard to the second alternative, reducing the metal temperatures experienced by the tip shroud by reducing the operating temperature of the gas turbine also is an undesirable solution. As one of ordinary skill in the art would appreciate, a reduction in operating temperature of the turbine results in a reduction in turbine efficiency. However, reducing the metal temperatures experienced by the tip shroud by cooling it during operation could extend the useful life of the part.
Thus, there is a need for improved systems for cooling turbine blade tip shrouds such that the metal temperatures associated with the high temperature turbine environment are reduced. The reduction in metal temperatures then will allow the part to better withstand the increased mechanical stresses associated with tip shrouds of larger surface area (i.e., unscalloped tip shrouds). Such a system would allow the tip shroud to better operate in the high temperature environment of the turbine with no scallop or the smallest scallop possible. Further, if such a system could cool the tip shroud while also reducing the weight of the tip shroud, further improvements in efficiency could be realized.